1. Field of the Invention
This invention relates to electro-optic sampling oscilloscopes that are used for observation of waveforms of measured signals which are detected by electro-optic sampling probes.
This application is based on Patent Application No. Hei 10-70872 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
The electro-optic sampling oscilloscopes operate using electro-optic sampling probes (where the term of "electro-optic sampling" can be abbreviated by "EOS") based on the known phenomena as follows:
Electric fields are produced by measured signals and are connected with electro-optic crystals, on which laser beams are incident. Based on polarization states of the laser beams, it is possible to measure waveforms of the measured signals. Herein, sampling operations are performed on the measured signals under the condition where the laser beams are made in form of pulses, so it is possible to measure the waveforms with a very high resolution.
The electro-optic sampling oscilloscopes (or "EOS oscilloscopes") have a variety of technical features and therefore attract attention to engineers, which is described in pp. 123-129 of the paper entitled "A High-Impedance Probe Based on Electro-Optic Sampling" written by Shinagawa and other members and published as the lectured monographs of the proceedings of 15.sup.th Meeting of the Institute of Lightwave Sensing Technology under the names of the Institute of Applied Physics and Institute of Lightwave Sensing Technology on May of 1995. The features of the EOS oscilloscopes are as follows:
(1) It is easy to perform the measurement on signals because the EOS oscilloscopes do not require ground lines. PA1 (2) A metal pin located at a tip end of the EOS probe is insulated from the circuitry, so it is possible to realize the high-input impedance which results in an ideal state that substantially no disturbances occur on states of the measuring points. PA1 (3) It is possible to actualize measurement of the broad frequency band up to GHz order because the EOS oscilloscopes use optical pulses for the measurement.
FIG. 10 is a block diagram showing an example of a configuration of the EOS oscilloscope, which is basically constructed by a main body 1 and a EOS probe 2 for receiving light (beams) corresponding to measured signals. Herein, a trigger signal TR is a periodically varying signal which synchronizes with clock pulses for driving a measured circuit (not shown) which is subjected to measurement. Therefore, the measured circuit supplies the trigger signal TR to the main body 1. Based on the trigger signal TR, a timing generation circuit 4 generates a pulse signal P1 for driving an optical pulse generation circuit 5 as well as a pulse signal P2 for driving an analog-to-digital converter (abbreviated by "A/D converter") 7. FIG. 11A is a time chart showing the trigger signal TR consisting of trigger pulses which periodically emerge; FIG. 11B is a time chart showing the pulse signal P1; and FIG. 11C is a time chart showing the pulse signal P2. Each of pulses of the pulse signal P1 is delayed from each of the trigger pulses of the trigger signal TR in such a way that a delay time "st" therebetween is gradually increased. In addition, each of pulses of the pulse signal P2 is delayed from each of the trigger pulses of the trigger signal TR by a prescribed and fixed delay time "dt".
The optical pulse generation circuit 5 receives the pulse signal P1 to generate optical pulses of laser beams, which are supplied to the EOS probe 2. When the optical pulse passes through the EOS probe 2, a polarization state thereof changes in response to a signal at a probe contact portion of the measured circuit. Then, the optical pulses whose polarization states are changed are converted to electric signals, which are supplied to a receiving light amplification circuit 6. The receiving light amplification circuit 6 amplifies output of the EOS probe 2 so as to produce a receiving light signal LS, which is then forwarded to the A/D converter 7. The A/D converter 7 performs sampling operations on the receiving light signal LS based on the pulse signal P2, so that the receiving light signal LS is subjected to analog-to-digital conversion. Then, a processing circuit 8 performs a display process of measured waveforms based on digital output of the A/D converter 7.
The aforementioned EOS oscilloscope suffers from a problem due to sampling of signals, as follows:
At sampling, external noise is input to the receiving light amplification circuit. Or, the EOS oscilloscope inevitably performs sampling operations on low-frequency noise components such as the noise (e.g., shot noise, heat noise and 1/f noise) caused by the receiving light amplification circuit and the optical noise (e.g., noise components on light beams produced by LEDs). Therefore, performance of the EOS oscilloscope is deteriorated in S/N ratio.